Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” was originally performed using wireline logging.
In wireline logging, an operator lowers a probe or “sonde” into the borehole after some or all of the well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, the sonde measures various parameters of the earth's formations and correlates them with the sonde's position as the operator pulls it uphole.
Although it is often useful, wireline logging does have its limitations. If the borehole has been cased, i.e., lined with steel casing that has been cemented in place, then the sensing abilities of most wireline tools are impaired. Typically the operator removes any tubulars in the borehole before performing a wireline logging run, thereby adding cost and delay to the logging process. Moreover, the delay often degrades the logging measurement quality due to migration of fluid from the borehole into the formation or caving and collapse of the borehole walls. Wall caving can potentially also trap the logging tool downhole.
Consequently, engineers have created other logging methods such as logging while drilling (“LWD”), and tubing conveyed logging. Such methods are unable to feasibly employ a logging cable because (if unprotected), the cable quickly gets pinched between the tubing and the borehole wall and sheared or shorted out. (Operators generally dislike employing cable protection measures because they require additional effort and they often fail anyway.) Thus engineers have created various alternative telemetry methods to communicate information between downhole tools and the surface. Such methods include mud pulse telemetry, acoustic telemetry, and very low-frequency electromagnetic telemetry. These methods demonstrate a limited range and/or a slow data rate. Recent development efforts have focused on tubing with integrated wiring, but the anticipated costs for such tubing remains prohibitively high due to the requirement for custom coupling designs.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to be limiting, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.